Cruz had 48 percent of the vote, compared with 35 percent for Trump and 14 percent for Ohio Gov. John Kasich. Cruz won 36 of the state’s 42 delegates, whereas Trump took just six -- narrowing his overall lead.

Cruz's showing in the Badger State is welcome news to establishment forces who have rallied behind the Texas conservative in an attempt to deny the controversial front-runner the necessary delegates to assume the party's nomination. Trump's inability to expand his delegate lead in Wisconsin also increases the chances of a contested convention later this summer in Cleveland.

Cruz led in eight of the last 10 polls of the Wisconsin GOP primary electorate. A Fox News poll last week showed him leading Trump by 10 percentage points. A smaller-sample poll released by ARG on Monday, however, found Cruz trailing Trump by the same margin. Cruz narrowly edged out Trump in the HuffPost Pollster average, which averages all publicly available polling data, as Wisconsinites headed to vote on Tuesday.

During a rally in Milwaukee, Cruz hailed his victory as a turning point that would ultimately lead him to secure his party's nomination later this summer.

"Tonight is a turning point, it's a rallying cry. It is a call from the hard-working men and women of Wisconsin to the people of America. We have a choice, a real choice," Cruz said, announcing that his campaign had raised over $2 million on Tuesday alone.

"As a result of tonight," he added, "I am more and more convinced that our campaign is going to earn the 1,237 delegates needed to win the Republican nomination. Either before Cleveland or at the convention in Cleveland, together we will win a majority of the delegates, and together, we will beat Hillary Clinton in November."

Trump did not appear publicly after his loss Tuesday evening, but his campaign released a statement in which he called Cruz a "Trojan horse" for the GOP establishment and accused the senator, without evidence, of illegally coordinating with his super PAC.

Cruz got a huge lift last week with the endorsement of Wisconsin Gov. Scott Walker, a popular figure in the state who ran for president before dropping out in September. The senator also enjoyed the backing of the state's evangelical leaders, a group that helped push him to victory in Iowa, as well as influential talk radio hosts who eviscerated Trump in days leading up to the primary.

In the last week of the campaign, Wisconsin airwaves were dominated by negative comments Trump made toward women. After threatening to "spill the beans" about Cruz's wife, Heidi, on Twitter, Trump retweeted an unflattering image of her alongside a flattering image of his wife, Melania. Cruz in turn called Trump a "sniveling coward," escalating the feud between the two men. (Over the weekend, Trump actually expressed regret for sharing the meme, stating on "Fox News Sunday" that “If I had to do it again, I probably wouldn’t have sent it.")

News that Trump's campaign manager was charged with simple battery after grabbing a female reporter during a campaign stop in Florida earlier this month also added fuel to the fire. So, too, did Trump's "throw everything at the wall and see what sticks" approach to the issue of abortion. After saying that women should be punished for illegal abortions in an interview last week, Trump attempted to walk back the assertion, and then promptly took several more nonsensical positions in an effort to smooth over anger from conservative pro-life groups.

Trump put on a confident air on the eve of the Wisconsin primary, predicting he would win just as he did in South Carolina -- where another popular governor, Nikki Haley, spurned his candidacy in favor of Sen. Marco Rubio.

"It was over, and I have a feeling the same thing’s gonna happen here. Because Walker has not done a great job," Trump said on Monday. "He has not done a good job. By the way, he’s been OK. You’re average. I hate to say it. You look around your surrounding states, you know what you are? Average! You’re not average people, right? So big deal. You’ve got the governor."

But in a rare sign of acknowledgement that his remarks toward women may have hurt his campaign, Trump enlisted his wife to speak alongside him during a campaign stop in Milwaukee on Monday, in hopes of blunting some of the criticism.

“No matter who you are, a man or a woman, he treats everyone equal,” Melania Trump said in brief remarks.

The remaining primary schedule is largely favorable to Donald Trump. The front-runner's blue-collar appeal could prove advantageous in his home state of New York, as well as in Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island, all of which vote in the month of April. Neighboring Massachusetts, which already voted on Super Tuesday, gave Trump his widest-margin victory and could be a harbinger of things to come for the mogul later this month.

The delegate-rich slate of states where Trump may dominate couldn't come at a worse time for the establishment wing of the GOP. A recent Washington Post-ABC News poll showed that if he secures the nomination, Trump would start the general election as the least popular major-party nominee in modern times, viewed overwhelmingly negatively by women, independents, Hispanics, and young adults.

This article was updated as more results became available.

CORRECTION: A previous version of this article incorrectly stated that there was an upcoming Republican contest in Wyoming. That contest was held in March.